The present invention relates generally to a differential gear unit for use in the middle of a power transmission path for rotationally driving the wheels of motor vehicles and, more particularly, to a differential gear unit fitted with a limited slip differential (LSD) for limiting the differential when a difference in the rotational speed occurs between a pair of output shafts provided in the differential gear unit.
In order to absorb a difference in the rotational speed between inside and outside wheels which inevitably takes place when automobiles turn, for example, a differential gear unit is conventionally disposed in the middle of a power transmission path. However, the typical differential gear unit involves a deficiency that once either a right or left drive wheel sticks in the mud or runs onto ice and snow on the surface of a road it starts to spin, the driving power is not to be transmitted to the drive wheel.
Thus, much use has been recently made of differential gear units equipped with a torque sensitive type limited slip differential for limiting the differential of the differential gear unit when a large difference arises in the driving torque between the right and left drive wheels, or with a speed sensitive type limited slip differential for limiting the differential of the differential gear unit in proportion to the difference in the rotational speed between the right and left drive wheels.
Among the differential gear units equipped with a torque sensitive type limited slip differential superior in operative response to and having a larger transmission driving force than the speed sensitive type, is a differential gear unit generally called a helical LSD or QUAIFE differential.
Description will be given of the differential gear unit of this type with reference to the accompanying drawings. Referring first to FIG. 4, a differential gear unit generally designated at 90 comprises a differential gear case 10, first and second side gears 31 and 32 serving as driven gears composed of large-diameter helical gears, first and second differential gears 35 and 36 composed of small-diameter helical gears, and a pair of output shafts 41.
The differential gear case 10 includes a substantially cylindrical body 11, and first and second closures 21 and 22 screwed with bolts 19 to the body 11 to close the openings at the ends thereof. The first and second closures 21 and 22 have substantially cylindrical bosses 23 and 24, respectively, integrally formed therewith. The bosses 23 and 24 are fitted into a bearing 2 supported by a differential gear carrier 1, the differential gear case 10 being rotatable on an axis C1.
Thus, the differential gear case 10 is rotated around the axis C1 when a rotational driving force from the engine is transmitted to a drive gear not shown secured to the differential gear case 10. The pair of output shafts 41 are rotatably supported around the axis C1 with their respective stems 42 being internally fitted into the bosses 23 and 24, respectively, and prevented from disengaging from the bosses 23 and 24 by means of circlips 49.
The first and second side gears 31 and 32 are carried within cylindrical openings 27 and 28, respectively, formed at the ends of the differential gear case 10 and are engaged with splined portions 43 of the pair of output shafts 41 so as to be also integrally rotated around the axis C1.
As is seen in FIGS. 4 to 6, the cylindrical body 11 of the differential gear case 10 is provided three sets of first and second cylindrical holes 12 and 13 which are accommodation sections for carrying the differential gears, each set being circumferentially equally spaced in a partially confronting manner, the holes 12 and 13 including axes C2 and C3, respectively, extending parallel to the axis C1 and equidistant from the axis C1. The internal diameters of the cylindrical holes 12 and 13 are slightly larger than the external diameters of the first and second differential gears 35 and 36, respectively, so that the first and second differential gears 35 and 36 carried within the first and second cylindrical holes 12 and 13 are supported rotatably around the axes C2 and C3, respectively.
The first and second differential gears 35 and 36 are partially meshed with each other and, as shown in FIGS. 5 and 6, their outer peripheral surfaces are partially exposed to the cylindrical openings 27 and 28, respectively, to allow the engagements with the first and second side gears 31 and 32.
Referring to FIGS. 5 and 6, the action of the differential gear trait 90 thus constructed will now be explained.
When the differential gear case 10 receives a driving force from the engine, the differential gear case 10 is rotated clockwise around the axis C1 as indicated by an arrow A in FIG. 5. This causes the first and second differential gears 35 and 36 carried within the cylindrical holes 12 and 13 respectively, of the differential gear case 10 to revolve in conjunction with the differential gear case 10 around the axis C1 in the direction indicated by arrows B and D. Since at that time the first and second differential gears 35 and 36 are meshed with the first and second side gears 31 and 32, the first differential gear 35 tries to rotate within the cylindrical hole 12 on the axis C2 in the direction indicated by a dotted arrow E, and in the same manner the second differential gear 36 tries to rotate within the cylindrical hole 13 on the axis C3 in the direction indicated by a dotted arrow F. However, the mutual engagement between the first and second differential gears 35 and 36 will prevent the first and second differential gears 35 and 36 from being independently rotated in the directions indicated by the dotted arrows E and F, respectively.
Thus, the first and second differential gears 35 and 36 revolve around the axis C1 in conjunction with the differential gear case 10 without being rotated on their respective axes C2 and C3, thereby causing the first and second side gears 31 and 32 meshed therewith to drivingly rotate on the axis C1 in the directions indicated by the arrows G and H, respectively.
On the contrary, when the automobile turns, a difference in rotational speed between the right and left drive wheels which is caused by the difference in turning radius between the inside and outside wheels must be absorbed by means of the differential gear unit. To absorb the difference in the rotational speed between the inside and outside wheels, in this instance, the differential gear unit 90 depicted in FIG. 4 is arranged such that the first and second differential gears 35 and 36 revolve around the axis C1 in the clockwise direction indicated by the arrows B and D while rotating on their respective axes C2 and C3 in the directions of the arrows I and J, respectively, as shown in FIG. 6, thereby drivingly rotating the first side gear 31 at a speed obtained by subtracting the amount of rotation of the first differential gear 35 from the number of rotations of the differential gear case 10, and drivingly rotating the second side gear 32 at a speed obtained by adding the amount of rotation of the second differential gear 36 to the number of rotations of the differential gear case 10.
Since this differential gear unit 90 employs the helical gears as the first and second differential gears 35 and 36 and the first and second side gears 31 and 32, the first and second differential gears 35 and 36 are not only biased radially outwardly with respect to the axes C2 and C3, respectively, by the engagement reaction force arising from mutual engagement with the first and second side gears 31 and 32, but also biased axially along the axes C2 and C3, with the result that the first mad second differential gears 35 and 36 are pressed and biased against not only the inner cylindrical surfaces 14 and 15, respectively, of the cylindrical holes 12 and 13, respectively, but also the bottom surfaces 16 and 17, respectively.
In the same manner, the first and second side gears 31 and 32 are thrusted axially along the axis C1 and are pressed and biased against the inner wall surfaces 25 and 26, respectively, of the first and second closures 21 and 22, respectively, of the differential gear case 10.
Accordingly, when either one of the right and left drive wheels sticks in the mud or runs onto ice and snow lying on the surface of the road to start to spin, there occurs a difference in the rotational speed between the first side gear 31 and the second side gear 32, allowing the first and second differential gears 35 and 36 to start to rotate within the cylindrical holes 12 and 13, respectively, of the differential gear case 10. At that time, the first and second differential gears 35 and 36 are thrust radially outwardly with respect to the axes C2 and C3, respectively, by the engagement reaction force arising from the engagement with the first and second side gears 31 and 32, respectively, and are pressed and biased against the inner cylindrical surfaces 14 and 15, respectively, of the cylindrical holes 12 and 13, respectively, of the differential gear case 10, resulting in a frictionally sliding rotation on their respective axes C2 and C3. Similarly, the first and second side gears 31 and 32 are thrusted axially along the axis C1 by the engagement reaction force arising from the engagement with the first and second differential gears 35 and 36, respectively, and are pressed and biased against the inner cylindrical surfaces 25 and 26, respectively, of the closures 21 and 22, respectively, of the differential gear case 10, resulting in a frictionally sliding rotation on the axis C1.
Thus, between the pair of output shafts 41 in this differential gear unit 90 there acts a limited slip differential torque caused both by a frictional resistance between the first and second differential gears 35 and 36 and the inner cylindrical surfaces 14 and 15, respectively, of the differential gear case 10 and by the frictional resistance between the first and second side gears 31 and 32 and the inner wall surfaces 25 and 26, respectively, of the differential gear case 10. This enables a driving force equal to this limited slip differential torque to be transmitted to the drive wheel opposite to the spinning drive wheel, making it possible to escape from the state where the wheel on one side has stuck in the mud or run onto ice and snow.
Nevertheless, in case either one of the right and left t drive wheels (e.g., the left-handed drive wheel in the diagram) has run onto ice of the ultra-low u road or come off to start to spin with substantially zero gripping force, the engagement three is not permitted to act between the first side gear 31 on the spinning wheel side and the first differential gear 35, reducing the transmission torque to zero. As a result of this, the first differential gear 35 is not pressed and biased against the inner cylindrical surface 14 of the differential gear case 10, and likewise the first side gear 31 is not pressed and biased either against the inner wall surface 25 of the differential gear case 10.
In this manner, the above-described differential gear unit 90 entails a problem that a driving force is not to be transmitted to the drive wheel since no limited slip differential torque occurs if the gripping force of the drive wheel on one side with respect to the road surface has thoroughly resulted in zero.